Foundations of Genetics

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Chapter 10

Foundations of Genetics
Lecture Notes

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Foundations of Genetics
Mendel and the Garden pea
The father of modern Genetics is Gregor Mendel. Gregor Mendel (1822-1884) was an Austrian monk who lived in a monastery where the experiments with the garden pea were performed. Mendel’s work with the garden pea was the fundamental study which unveiled the laws that govern genetics and heredity. Mendel was the first to use the scientific method in a very systematic and analysed his results and observations statistically.

We know that British farmers performed experiments with the Garden Pea much earlier before Mendel. One of these was T. A. Knight who in the 1790’s performed crosses with purple and white flowers and made the observations that some traits have “stronger tendency” to show than others. However, Mendel counted every flower and offspring that exhibited a trait. Mendel also performed very detailed statistical analysis and carefully documented the results and mathematical relationships from one generation to the next. Gregor Mendel’s experiments and the results obtained allowed him to come up with simple but very powerful hypothesis. His predictions and explanation turned out to be scientifically significant and made modern genetics a reality. We now know that hereditary traits are specific and precise instruction laid out in the DNA of the parents.

Mendel’s Experimental systems
The choice of a garden pea to study genetics was appropriate because there were several characteristics that are very desirable. These include: 1. The pea has several varieties of which Mendel selected seven that had easily detectable characteristics or traits. 2. Mendel knew from previous work like Knight, some of the traits did not show in one generation or the other. 3. Peas are easy to study, mature quickly and small and therefore easily manipulated. 4. The flowers of peas are inside and if left alone do not open and fertilize themselves (with their own pollen).

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Mendel’s Experimental design
Mendel used the same method that Knight in the 1790’s. The key difference he introduced was that he counted all the offspring very carefully. In his studies Mendel followed the following steps. 1. Mendel let each pea variety to self fertilize for several generations to make sure that each variety he was working with is true-breeding (meaning, it produced only red flowers or they were all tall etc…). 2. Mendel then crossed two peas showing different traits, for example a pea producing white flowers were crossed with peas that were producing red flowers. The offspring from this generation was the F1 (first filial) generation. 3. Mendel then took the peas produced from the F1 generation and let them self fertilize to obtain the F2 (second filial) generation and counted the offspring.

Mendel’s Experiments

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Mendel repeated his crosses with several traits such as the color of lowers, color of seeds, and color of the pods and carefully counted the offspring and came out with the statistical predictions.

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Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display

Mendel’s Observations
Mendel experimented with seven traits in total (color of flower, color of seeds, color of pods, etc…). The results obtained from all of the experiments showed very similar results in terms of proportions. Here is a summary of what he found: • F1 generation: when Mendel crossed true-breeding purple flowered peas with white flowered peas all of the flowers were purple. This shows that purple is the dominant trait (trait that was expressed in F1) and white is the recessive trait (the trait that was not expressed in F1). F2 generation: when Mendel then self fertilized the purple flowered peas he obtained 75% of the flowers were purple and 25% of them were white. In other words 3:1 ratio.



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